Field of the Invention
The present invention relates to a non-volatile memory, and more particularly to a magnetic random access memory (MRAM) and a method for programming the same memory subjected to high temperature exposure.
Description of the Prior Art
Certain types of non-volatile memory, such as magnetic random access memory (MRAM) and phase change magnetic random access memory (PCRAM), may lose saved data when exposed to high temperatures during manufacturing. Unlike flash memory, memory cells of MRAM and PCRAM with valid saved data may not retain the saved data when exposed to high temperatures that may cause unintended switching of the resistance state of the memory cells.
During manufacturing of MRAM with memory cells made of magnetic tunnel junctions (MTJs), test code, boot code, and the like are oftentimes written to the memory at the wafer level prior to dicing and packaging, which may expose the MTJ memory cells to temperatures of greater than 200° C. For example, upon completion of cutting and sorting of numerous dies of a wafer during manufacturing, each die is packaged. Packaging may expose the MTJ memory cells in a die to high temperatures. Similarly, soldering of an MRAM die to a circuit board would require heating the solder to cause reflow, thereby exposing the MTJ memory cells in the die to high temperatures. If certain data is written to the MTJ memory cells of the die before the solder reflow process, which may expose the MTJ memory cells to temperatures of 200° C. and above for periods of 2 minutes and longer, the written data may become corrupted as the MTJ memory cells randomly switch their resistance state after the reflow process.
FIG. 1 shows a graph 10 of resistance distributions of a population MTJs being built during manufacturing using current manufacturing techniques. It is first noted that the behavior of MTJs is similar to resistors; accordingly, resistance or resistance value as referred to herein, represents the resistance of MTJs.
In FIG. 1, the graph 10 shows resistance in the x-axis and the number of MTJs in the y-axis. Accordingly, two resistance distributions are shown by the graph 10, one being the resistance distribution 12 of the MTJs at a state represented by high resistance (RH) and another being the resistance distribution 14 of the MTJs at a state represented by low resistance (RL).
The gap shown between the inner edges of the resistance distributions 12 and 14 distinguishes one state from another during read and write operations. For example, the resistance range shown at 16 in graph 10 separates the RH from RL distributions. A resistance within the distribution curve 14 could be considered (or sensed) a “high” state, i.e. logical value “1”. Similarly, a resistance within the distribution curve 12 could be considered or sensed as a low state, i.e. “0”. It is noted that the state representations may be reversed.
The resistance state of an MTJ memory cell may randomly switch to an opposite state when exposed to high temperatures, such as those encountered during packaging or solder reflow process. This become a problem when manufacturers save data to MRAM before exposure to high temperatures.
Currently, there are no known solutions for the foregoing issue. It is noted that MRAM is not the only type of non-volatile memory suffering from the foregoing problem. Indeed, any non-volatile memory that is exposed to high temperatures for a relatively lengthy period of time may similarly suffer from the data corruption problem.